The two have had a rivalry since childhood, when Hungary believed herself to be a boy (as did Prussia). It is said that she loathes Prussia due to him making fun of her hardships since that time. She is also known to hit him with her frying pan when she catches him bothering Austria, and had appeared at his bedside fully armed with a spear to threaten him to give back Silesia after he had seized it.
Prussia had also noticeably groped Hungary when they were children, though he was unaware of her true gender at the time and simply thought he'd grab a weak spot. After realizing the truth, he became disgusted and upset that he might have committed a sin, and was utterly horrified by his actions. His realization led him to confessing his "sin" in a chapel and begging God for forgiveness.
Informations Source: Hetalia Wikia